I have a QNAP TS-469L with 4x WD Red 3TB in RAID6. I get anywhere between 60-80MB/s sustained depending on the files being copied and bursts up to full gigabit speed (112MB/s give or take). Performance was far worse with the previous version of QTS (3.6.x I believe). QTS 4.x brings huge improvements in both performance and the web layout.

No stability issues thus far to speak of. Right now I'm sitting at 37 days uptime, though I've had it since last Christmas. I usually reboot everything in my house roughly once a month though.

From what I gathered when I settled on this model, all the ARM-based units will not achieve the performance you want. You'll definitely need an Atom-based unit instead.